A wireless communication system is a cellular system. The cellular system means a system which provides a communication service by dividing a service zone into a plurality of cells. Through the cellular system, it is possible to overcome restriction on the service zone and a limit on capacity for users. The cell is a specific geographical zone where one base station BS provides communication service. A terminal belongs to one cell and connects with the BS of the cell, thereby communicating with the base station.
In the wireless communication system, a mobile terminal has to continuously communicate with the BS. In the case that the terminal moves from one cell to another cell, the terminal has to disconnect from the existing BS and newly connect with a BS that belongs to a new cell. Thus, a procedure of changing the BS due to the movement of the terminal is called a handover. Here, the BS to which the terminal is currently connected will be called a service BS, and the BS with which the terminal tries to newly connect will be called a target BS.
The wireless communication system may be a single-carrier system that supports one carrier. In the single-carrier system, one carrier is used for data exchange between the BS and the terminal. Since a transmission rate is proportional to a transmission bandwidth, the transmission bandwidth has to be increased to support a high transmission rate. However, frequency assignment for a large bandwidth is not easy except some areas from all over the world. To efficiently use a split small band, spectrum aggregation has been developed. The spectrum aggregation is also called bandwidth aggregation or carrier aggregation. The spectrum aggregation groups a plurality of physically contiguous or non-contiguous bands in a frequency domain and thus has the same effect of logically using a large band. Through the spectrum aggregation, multi-carriers can be supported in the wireless communication system. The wireless communication system capable of supporting the multi-carriers is called a multi-carrier system. In the multi-carrier system, one or more carriers are employed for the data exchange between the BS and the terminal.
However, the multi-carrier system is different in the handover from the single-carrier system. Accordingly, an efficient method for the handover is needed in the wireless communication system that supports the multi-carriers.